As a DM, I cheat.
It got worse since the 4E announcement. my Iron Heroes monster sometimes have double or half hit points, deal 50 % more damage, and stuff like that.
During a session, I will "adjust" dice rolls if I feel the encounter is going to easy or too hard on the PCs.
As a player, I cheat seldom and always with a bad conscience. But I do.
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I don't have a girlfriend at the moment, but I do not believe I would cheat on her. I am to single-minded...